1. Field of the Invention
The invention relates to a spark plug mounted on an internal combustion engine, and particularly concerns to a spark plug which is improved to facilitate the self-cleaning action against the carbon-related deposit collected on an insulator.
2. Description of Prior Art
In a semi-creeping discharge type spark plug disclosed by e.g., U.S. Pat. No. 5,448,130, an air gap is formed between an outer surface of a center electrode and a front end surface of a ground electrode, and releasing creeping spark discharges along a front end surface of an insulator so as to facilitate the self-cleaning action.
As an extension technique of the above spark plug, an intermittent creeping discharge type spark plug has been introduced in which a spark discharge gap is formed between an outer surface of the center electrode, and creeping spark discharges are released from an inner edge portion of a ground electrtode toward a front end surface of an insulator so as to facilitate the self-cleaning action when the insulator is carbon fouled.
However, the insulator nose is likely to be carbon fouled especially when running the engine at the time of traffic congestion in winter seasons. This often leaks a high voltage through the carbon deposit so as to induce a flashover phenomenon in which the spark discharges tend to irregularly jump deep behind an open-ended metal shell.
The flashover phenomenon prevents the spark discharges from normally running across electrodes, thus inviting inconveniences such as, for example, an engine stall, unstable idling, loss of cold starting capability and insufficient accelaration of the engine. In order to remedy these inconveniences, it has been desired to introduce effective countermeasures against the flashover.
As if to make the situation get worse, a ledge portion is provided at an inner wall of the metal shell to project inward so as to rest an insulator thereon by way of a shoulder portion. The presence of the ledge portion diminishes a distance between the ledge portion and an outer surface of the insulator, so as to intensify an electrostatic field around an edge of the ledge portion so as to induce the flashover toward the ledge portion when insulator is carbon fouled unacceptably.
In order to increase the distance between the ledge portion and the outer surface of the insulator, it is supposed to diametrically thin the insulator. This, however,increases a front open area of the metal shell to often invite an entry of carbon (soot) therethrough so as to deteriorate a fouling resistant property.
Therefore, it is a first object of the invention to provide a semi-creeping discharge type spark plug which is capable of effectively protecting the ledge portion against the flashover even when the insulator nose is unacceptably carbon fouled, and further maintaining a high insulation resistance by facilitaing the self-cleaning action due to the creeping spark discharges.
It is a second object of the invention to provide an intermittent creeping discharge type spark plug which is capable of effectively protecting the ledge portion against the flashover even when the insulator nose is carbon fouled considerably, and further insuring a high insulation resistance by facilitaing the self-cleaning action due to intermittent creeping spark discharges.